Attenuation of Noise by Motorcycle Safety Helmets

8/12/2019 Attenuation of Noise by Motorcycle Safety Helmets

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International Journal of Occupational Safety and Ergonomics (JOSE)2009, Vol. 15, No. 3, 287293

This study was partly supported by grant IV-11 from the Ministry of Labour and Social Affairs of Poland and by grant COST/199/2006from the Ministry of Science and Higher Education of Poland.

Correspondence and requests for offprints should be sent to Rafa Myski, Central Institute for Labour Protection National ResearchInstitute, Czerniakowska 16, 00-701 Warszawa, Poland. E-mail: .

Attenuation of Noise by Motorcycle

Safety Helmets

Rafa MyskiEmil Kozowski

Central Institute for Labour Protection National Research Institute (CIOP-PIB), Poland

Jan era

Central Institute for Labour Protection National Research Institute (CIOP-PIB), PolandInstitute of Radioelectronics, Faculty of Electronics and Information Technology,

Warsaw University of Technology, Warszawa, Poland

For workers such as police motorcyclists or couriers, traffic and engine noise reaching the ears is an

important factor contributing to the overall condition of their work. This noise can be reduced with

motorcycle helmets. In this study, insertion loss of motorcycle helmets was measured with the microphone-in-

real-ear technique and sound attenuation with the real-ear-at-threshold method. Results for 3 Nolan helmets

show essentially no protection against external noise in the frequency range 500 H, ttenutn ncree lnerl t rte f 89 dB per ctve, t ~30 dB t 8 H. Lc fattenuation in the low-frequency range may cause annoying effects. In addition, high attenuation in the high-

frequency range may decrease intelligibility of speech signals for a rider in a helmet. Attenuation measured in

this study does not take into account noise generated by turbulent wind around the helmet. Thus, the measured

vlue f ttenutn repreent mtrccle rder bet cndtn f herng.

noise attenuation measurements on subjects motorcycle helmets

motorcyclist conditions of work

1. INTRODUCTION

There are several occupations including police

officers, couriers and letter carriers, in which

motorcycles are widely used. Motorcycle safety on

a road involves several issues such as conspicuity

of riders and their vigilance and alertness.

Whereas conspicuity is a general problem relatedto the visibility of the motorcycle (including the

motorcycle itself, the drivers uniform and thehelmet), a drivers alertness may be affectedby the limitations in the perception of signals

associated with the road conditions as well as

poor physiological conditions under the helmet.

Among factors degrading physiological conditions

under the helmet such as excessive concentration

of carbon dioxide, excessive heat and humidity orlimited vision, noise is another factor contributing

to the conditions of riding the motorcycle [1], and

thus the working conditions of the occupationalgroups just mentioned [2]. Motorcyclists at

work, including policemen and letter carriers, areexposed to noise levels >100 dB(A) [3].

Sound reaching a motorcyclists ear can besubdivided into components including noise

generated by wind turbulences around the helmet


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288 R. Myski, E. kozowski & J. ERa

JosE 2009, Vl. 15, N. 3

and sounds generated on the road such as the

motorcycle engine and tyre noise. Noise definitely

impedes the conditions of driving and interferes

with other sounds coming from the road that may

contain useful information or are part of warningsignals necessary in preventing accidents. Thus,

the motorcycle helmet has to be treated as a device

attenuating noise and useful signals generated on

the road, and at the same time as a generator of

turbulent noise. The latter becomes an important

factor for driving speeds above ~50 km/h.In this study, attenuation of outside sounds

provided by motorcycle helmets was measured

using methods similar to those commonly applied

to hearing protectors. In terms of a measurementmethod, a motorcycle helmet was treated like ahearing protector that offers protection against

road noise but unfortunately also attenuates

signals bearing useful information on various

road conditions.

For labelling purposes, sound attenuation of

hearing protective devices such as earmuffs or

earplugs is measured as a difference in level

measured at the threshold of hearing when the

protector is donned or doffed. This subjectivemeasurement called real ear at threshold (REAT)

is required by Standard No. EN 24869-1:1992[4]. This procedure can be used for determining

sound attenuation of a motorcycle helmet.

Measurement of insertion loss introduced by a

helmet is an objective method alternative to the

subjective method from Standard No. EN 24869-

1:1992 [4]. It was previously used to determinethe insertion loss of shotblasting helmets [5, 6]. In

those studies, helmets were positioned on head-

and-torso simulators or human subjects. Insertion

loss was measured using a miniature microphone

fixed in the concha. Results for shotblasting

helmets showed that attenuation introduced by

helmets was negligible for frequencies


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289NoisE aTTENUaTioN By MoToRCyCLE HELMETs

JosE 2009, Vl. 15, N. 3

3. MEASUREMENT SITE AND

PROCEDURE

Measurements were performed in a test room

originally designed for determining sound

attenuation of hearing protectors (Figure 4). The

diffused sound in the area of a subjects headposition was produced with four JBL (USA) 4802loudspeakers powered by two QUAD (UK) 520fpower amplifiers. The test room and the diffused

sound field met the requirements of Standard No.EN 24869-1:1992 [4]. Pink noise was generatedwith a Norsonic (Norway) type 828 system

controlled with a PC. The wide band level of pinknoise (frequency range 5010 000 Hz) was set to97 dB. The subjects were seated in the middle oftest room. The average height of the entrance of

the ear canal was 1.19 m above the floor.

3.1. Insertion Loss Measurements

For insertion loss measurements, the noise

level was recorded with a miniature Knowles(USA) BL 1785 microphone. It was placed atthe entrance of the subjects ear canal and wasattached to the subject with medical tape to avoid

any movements when the helmet was donned or

doffed. Recorded signal was delivered to a Svan

948 spectrum analyser (Svantek, Poland) andanalysed in third-octave bands. The measurement

system was checked with a Brel & Kj

r(Denmark) type 4230 calibrator. Calibration wasperformed before and after each measurement

session and every time when the microphone was

removed from its position at the entrance of the

ear canal.

A measurement session comprised two steps.

First, the subject was seated, and the microphone

was placed at the entrance to the right ear

canal. The measurements in third-octave bands

were conducted for an uncovered ear. In thesecond step, the levels were determined at the

same position of the microphone at the ear

canal entrance, but the ear was covered with a

motorcycle helmet. Insertion loss measurements

were made for each of the helmets tested, on 3,

5 and 8 subjects for Diamond N41 XL, N70 XLand N102 XL Nolan helmets, respectively. The

subjects were 2852 years old. Their age wasnot a critical parameter as the test was merely an

acoustical measurement of the sound pressurelevel and the subjects did not perform any active

role during the session.

For each condition, equivalent levels were

determined using linear time averaging of

Figure 3. An N70 XL helmet (Nolan, Italy).

Figure 4. A subject in the measurementchamber.


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JosE 2009, Vl. 15, N. 3

the Svan 948 spectrum analyser over 30 s.

Measurements were repeated 3 times for each

condition. The subjects were given sufficient

rest between measurements. Short averaging

time and the measurement procedure ensuredthat subject were not exposed to over normative

doses of noise. Insertion loss was calculated as a

difference between third-octave levels measured

at the entrance of an uncovered ear and levels

measured under the helmet.

3.2. REAT Measurements

Attenuation of helmets with the REAT procedure

was done according to Standard No. EN 24869-

1:1992 [4], except for the number of subjects.Two subjects, a man (32 years old, subject 1) and

a woman (28 years old, subject 2) participated in

the measurements. Their audiometric thresholds

complied with the requirements in the standard.

Centre frequencies of one-third octave band-pass

noise were spaced in octaves from 63 to 8 000 Hz.

Threshold estimates were made for the helmet

donned and doffed interchangeably, and sound

attenuation was determined as the differencein thresholds between those two measurement

conditions. All other testing conditions such

as details of the procedure for determining the

threshold and noise floor levels in the testing

room followed Standard No. EN 24869-1:1992.The measurements were performed for the Nolan

N70 XL helmet.

4. RESULTS

Figure 5 illustrates the insertion loss of three

motorcycle helmets defined as the difference

in signal levels in third-octave bands measured

with helmets doffed and donned. Regardless

of the details of the construction of the helmets,

there was essentially no attenuation of the outer

noise level for frequencies


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JosE 2009, Vl. 15, N. 3

Figure 5. Insertion loss of Nolan (Italy) motorcycle helmets tested on subjects with MIRE (microphone

in real ear); (a) Diamond N41 XL, (b) N102 XL, (c) N70 XL. Notes.Various symbols represent the spreadof individual results.Solid line represents average insertion loss.


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JosE 2009, Vl. 15, N. 3

7. CONCLUSIONS

External noise attenuation provided by the

motorcycle helmets is largely frequency

dependable. In the low-frequency range (up

to ~250 Hz), helmets provide essentially no

protection against external noise. In the frequency

range >500 Hz, attenuation increases linearly at arate of 89 dB per octave, to ~30 dB at a frequencyof 8 kHz. This attenuation characteristic is similarregardless of detailed differences in the design of

the helmets, and is similar to the attenuation of

shotblasting helmets [6]. The pattern of motorcycle

helmet attenuation obtained in this study suggests

two unfavourable effects related to helmet use.

Poor attenuation in the low-frequency range results

in a negligible decrease in low-frequency annoying

sounds that may cause riders fatigue and lowalertness. High attenuation in the high-frequency

range may seriously impede riders understandingand comprehension of speech.

Sound attenuation measured with the REAT

method is generally similar to attenuation

determined as insertion loss. Nevertheless, there

Figure 6. Insertion loss (mean values) of 3 Nolan (Italy) motorcycle helmets tested on subjects withMIRE (microphone in real ear).

Figure 7. Sound attenuation measured on 2 subjects with REAT method (Nolan N70 XL). Notes.Solid lines show insertion loss measurements on the same subjects; REATreal ear at threshold, MIRE

microphone in real ear.


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JosE 2009, Vl. 15, N. 3

are certain effects related to breathing that affect

measurement results. This additional noise caused

by breathing may occur during a normal use of

a helmet. Therefore, elevated attenuation in the

low-frequency range observed for one subjectmay be related to additional masking of usefulexternal signals during actual helmet use.

Attenuation measured in this study does not

take into account noise generated in the helmetby turbulent wind during a motorcycle ride at a

sufficiently high speed. This noise will cause

additional masking of useful warning signals andspeech coming from external sources. Therefore,

measured attenuation represents best riders

conditions of hearing during a ride when thehelmet is worn.

REFERENCES

1. Lower MC, Hurst DW, Claughton AR,Thomas A. Sources and levels of

noise under motorcyclists helmets. In:Proceedings of the Institute of Acoustics.

Vol. 16. St. Albans, Herts., UK: Institute ofAcoustics; 1994. p. 31926.

2. Lower MC, Hurst DW, Thomas A. Noiselevels and noise reduction under motorcycle

helmets. In: Proceedings of Internoise 96.Indianapolis, IN, USA: Institute of NoiseControl Engineering; 1996. p. 97982.

3. Ross BC. Noise exposure of motorcyclists.Ann Occup Hyg. 1989;33(1):1237.

4. European Committee for Standardization(CEN). Sound attenuation of hearing

protectors. Part 1: Subjective method ofmeasurement (Standard No. EN 24869-

1:1992). Brussels, Belgium: CEN;1992.

5. Patel J, Irving F. Measuring the noise

attenuation of shotblasting helmets. Ann

Occup Hyg. 1999;43(7):47184.

6. era J, Myski R. Attenuation of outsidenoise by shotblasting helmets. Arch Acoust.

2004;29(2):34758.

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Attenuation of Noise by Motorcycle Safety Helmets